In this proposal our primary objective is to continue experiments designed to elucidate the effects of use (exercise-training) and disuse on fast and slow skeletal muscle, and determine to what extent and by what mechanism exercise-training protects against the development of fatigue. Our long-term objective is to understand the molecular mechanisms controlling muscle function such that this knowledge can be utilized in the prevention of fatigue and disease, and in rehabilitative medicine. The effect of exercise-training and long-term disuse on muscle function, and the etiology of muscle fatigue, will be studied in the rat. The physiological studies will include an in situ and in vitro evaluation of the contractile properties of the slow-twitch soleus, and fast-twitch extensor digitorum longus (EDL), at rest and during acute contractile activity. The intracellular H+ ion concentration will be directly measured using pH-sensitive glass microelectrodes, and pH changes during work determined. The pH changes will be correlated to alterations in substrate levels and contractile properties to assess the role of the H+ in muscle fatigue. The skinned fiber preparation will be utilized to determine how exercise-training, fatigue, and disuse affect sarcoplasmic reticulum (SR) and myofibrillar function. The contribution of myofibrillar and SR ATPase to the total activity will be determined and these activities correlated to maximal shortening velocity and force transients respectively. The sarcoplasmic reticulum (SR) will be studied to determine how exercise-training, muscle fatigue, and disuse affect the kinetic properties, phosphorylation, and gel electrophoretic patterns of this membrane system. These experiments are designed to test our hypothesis that the SR controls the intensity and duration of the active state, while the maximal speed of unloaded shortening is dependent on the myosin ATPase activity.